Color has always played an important role in the life of all organisms on Earth. Human life has become truly “colorful” due to the use of colors in all its aspects, including clothes, food, and furniture. Much archaeological evidence has shown that the use of pigments as coloring agents has been practiced since ancient times [1]. Pigments, especially synthetic ones, have occupied the entire market due to their wide range of applications in different industries since their discovery in the 19th century. Different attributes such as low production costs, ease of production, and superior coloring properties have largely contributed to the establishment of synthetic pigments in the market. However, the use of synthetic colors has been found to be detrimental to human health and the environment because of their many adverse impacts ^{[2][3][4][5][6][7]}[2,3,4,5,6,7]. Many disadvantages of synthetic pigments, such as poor degradation, longer persistence, potential to cause cancers/allergies, etc., have increased the demand for natural, organic, and eco-friendly pigments in the current era.

The global response, as well as the demand for eco-friendly natural pigments, has significantly increased in recent decades due to their advantages over hazardous synthetic pigments. They are used as colorants, color intensifiers, additives, antioxidants, etc., in many industries including the textile, pharmaceutical, cosmetic, painting, food, and beverage industries ^[1][8][1,8].

2. Natural Pigments

Natural pigments are naturally derived pigments synthesized mainly by plants, animals, and microbes ^[5][9][5,9]. Most of the natural pigments used for different purposes since ancient times are produced from plants, such as annatto, grapes, indigo, beetroot, turmeric, madder, saffron, etc. ^[10][11][10,11]. However, the process of pigment production from plants may not be a good option because of various problems, such as season dependency, loss of vulnerable plant species due to their extensive use, variations in color shades and intensity, expensive production, and issues related to stability and solubility [2]. Nowadays, microorganisms, including bacteria, fungi, and algae, have been shown to be an excellent alternative source of natural pigments. For the large-scale production of pigments, microorganisms are more suitable, due to a clear understanding of their cultural techniques, processing, and ease of handling. Natural pigments from microbes, especially from bacteria and fungi, have been reported worldwide by many researchers ^{[1][10][12][13][14][15][16][17][18][19][20]}[1,10,12,13,14,15,16,17,18,19,20]. Many bacterial species have been reported to possess potential for pigment production ^{[10][21][22][23]}[10,21,22,23], but their pathogenic nature as well as associated toxicity have blocked production and commercialization. This eventually opened a new avenue for producing pigments from fungi and for their various applications.

3. Fungal Pigments

Fungi have been shown to be a good and readily available alternative source of natural pigments ^{[1][20][24][25][26]}[1,20,24,25,26]. Fungi have immense advantages over plants such as season-independent pigment production, easy and fast growth in a cheap culture medium, production of pigments with different color shades and of more stable, soluble pigments, and easy processing ^[10][27][10,27]. Fungi belonging to the Monascaceae, Trichocomaceae, Nectriaceae, Hypocreaceae, Pleosporaceae, Cordycipitaceae, Xylariaceae, Chaetomiaceae, Sordariaceae, Chlorociboriaceae, Hyaloscyphaceae, Hymenochaetaceae, Polyporaceae, Ophiostomataceae, Tremellaceae, Herpotrichiellaceae, and Tuberaceae families have been described as potent pigment producers ^{[8][12][20][25][26][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]}[8,12,20,25,26,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45]. These fungi are known to synthesize a variety of pigments as secondary metabolites. They are prolific producers of pigments belonging to several chemical classes, such as carotenoids, melanins, azaphilones, flavins, phenazines, quinones, monascin, violacein, indigo, etc. ^{[16][25][26][46][47][48][49]}[16,25,26,46,47,48,49]. The use of Monascus pigments for the production of red mold rice (ang-kak) is the oldest recorded use of fungal pigments by humans. Certain species of Monascus, viz., Monascus ruber and Monascus purpureus, have been reported to be good potential producers of pigments worldwide. Studies have shown the potential of the red pigment produced by M. ruber as an important food colorant as well as food additive ^[50][51][50,51]. Many new pigments produced by M. ruber, such as N-glucosylrubropunctamine, N-glucosylmonascorubramine, monarubrin, rubropunctin, etc., have been discovered (Figure 1) ^[52][53][54][52,53,54]. Recently, researchers revealed the first detailed biosynthetic pathway of Monascus azophilone pigments (MonAzPs) in M. ruber M7, based on targeted gene knockouts, heterologous gene expression, as well as in vitro enzymatic and chemical reactions [55]. Along with M. ruber, M. purpureus was also reported to produce a variety of novel pigments, such as monapurone A–C, monasphilone A–B, monapilol A–D, and 9-(1-hydroxyhexyl)-3-(2-hydroxypropyl)-6a-methyl-9,9a-dihydrofuro[2,3-h] isoquinoline-6,8 (2H,6aH)-dione (Figure 1) ^{[56][57][58][59]}[56,57,58,59]. Another study reports on the physicochemical (pH, light, and heat stability) properties of the red pigment of M. purpureus [60]. Along with Monascus, many species of Fusarium have been reported for their capability to produce pigments. Studies have reported pigments such as bikaverin, nor-bikaverin, fusarubins, some naphthoquinone (8-O-methybostrycoidin, 8-O-methylfusarubin, 8-O-methylnectriafurone, 8-O-methyl-13-hydroxynorjavanicin, 8-O-methylanhydrofusarubinlactol, and 13-hydroxynorjavanicin), and a novel isoquinoline-type, pigment 2-(4-((3E,5E)-14-aminotetradeca-3,5-dienyloxy)butyl)-1,2,3,4-tetrahydroisoquinolin-4-ol (ATDBTHIQN), from Fusarium fujikuroi (formerly known as Fusarium moniliforme) (Figure 2) ^[25][61][62][25,63,65]. Similarly, differently colored naphthoquinones [bostrycoidin, 9-O-methylfusarubin, 5-O-methyljavanicin, 8-O-methylbostrycoidin, 1,4-naphthalenedione-3,8-dihydroxy-5,7-dimethoxy-2-(2-oxopropyl), 5-O-methylsolaniol, and 9-O-methylanhydrofusarubin], two anthraquinones compounds [2-acetyl-3,8-dihydroxy-6-methoxy anthraquinone and 2-(1-hydroxyethyl)-3,8-dihydroxy-6-methoxy anthraquinone], and polyketide pigment (bikaverin) were reported from Fusarium oxysporum (Figure 2) ^{[25][47][63][64]}[25,47,64,67]. Another species of Fusarium, Fusarium graminearum, has been found to produce a variety of pigments such as 5-deoxybostrycoidin anthrone, 6-O-dimethyl- 5-deoxybostrycoidin anthrone, purpurfusarin, 6-O-demethyl-5-deoxybostrycoidin, 5-deoxybostrycoidin, and aurofusarin (Figure 2) ^{[25][63][65][66]}[25,64,66,121]. A red pigment aurofusarin has been found to be produced by many species of Fusarium such as Fusarium culmorum, Fusarium sporotrichioides, Fusarim. acuminatum, Fusarium avenaceum, Fusarium poae, Fusarium crookwellens, Fusarium pseudograminearum, Fusarium sambucinum, and Fusarium tricinctum. Bikaverin has been reported to be produced by Fusarium lycopersici, and Fusarium vasinfectum. Fusarium solani and Fusarium verticillioides (currently known as F. fujikuroi) have been described to produce both aurofusarin and bikaverin (Figure 2) [25]. Similarly, benzoquinone has been reported from Fusarium sp. JN158 (Figure 2) ^[67][68]. A study has shown that the synthesis of major Fusarium carotenoids (neurosporaxanthin and β-carotene) is induced by light via transcriptional induction of the structural genes carRA, carB, carT, and carD [43]. Similarly, other members of the fungal family Nectriaceae, such as Albonectria rigidiuscula and Fusicolla aquaeductuum (formerly known as Fusarium decemcellulare and Fusarium aquaeductuum respectively) were reported for their pigment production potential (Figure 2) ^[43][63][43,64]. Recently, the biosynthetic pathway of chrysogine mediated by two-module non-ribosomal peptide synthetase (NRPS) gene cluster was discovered in Fusarium graminearum in which enhanced chrysogine production was observed upon overexpression of NRPS14 ^[68][122]. Many investigations report Penicillium as potent producers of pigment ^{[25][69][70][71][72]}[25,61,96,97,98], such as arpink red^TM (first commercial red colorant), talaroconvolutins A–D, sclerotiorin, xanthoepocin, atrovenetin, and dihydrotrichodimerol discovered from Penicillum oxalicum var. armeniaca, Penicillum convolutum (formerly known as Talaromyces convolutes), Penicillum mallochii, Penicillum simplicissimum, Penicillum melinii, and Penicillum flavigenum, respectively (Figure 3a) ^{[41][73][74][75][76]}[41,91,93,94,123]. An uncharacterized red pigment has been reported from Penicillium miczynskii ^[77][71]. Besides, many other Monascus-like pigments such as PP-V [(10Z)-12-carboxylmonascorubramine] and PP-R [(10Z)-7-(2-hydroxyethyl)-monascorubramine] have been reported from Penicillium (Figure 4) ^[78][95]. A biosynthetic pathway for the yellow pigment chrysogine from Penicillium chrysogenum has been proposed recently ^[79][92]. Talaromyces spp. have been reported as a source of pigments by many researchers. The pigment production ability of Talaromyces purpureogenus (formerly known as Penicillium purpureogenum) was evaluated by many researchers ^[85][86][87][102,104,105]. Studies report the production of a herqueinone-like pigment from Talaromyces marneffei (formerly known as Penicillium marneffei), Monascus-like azaphilone pigments (N-glutarylmonascorubramine and N-glutarylrubropunctamine) from Talaromyces purpureogenus (formerly known as Penicillium purpureogenum), industrially important red pigments (mitorubrin, monascorubrin, PP-R, glauconic acid, purpuride, and ZG-1494α) from Talaromyces atroroseus, trihydroxyanthraquinones (emodin, erythroglaucin, and catenarin) from Talaromyces stipitatus, and a xanthone dimer (talaroxanthone) from Talaromyces sp. (Figure 3b) ^{[80][81][82][83][88]}[100,101,103,107,109]. An uncharacterized red pigment was discovered from Talaromyces siamensis under submerged fermentation ^[77][71]. Moreover, other species of Talaromyces, Talaromyces aculeatus, Talaromyces atroroseus, Talaromyces albobiverticillius, Talaromyces cnidii, Talaromyces coalescens, Talaromyces pinophilus, Talaromyces purpurogenus, Talaromyces funiculosus, Talaromyces amestolkiae, Talaromyces ruber, Talaromyces stollii, and Talaromyces verruculosus have been reported to have the ability to produce Monascus-like azaphilone pigments (Figure 4) ^[25][84][25,106]. Several members of the genus Aspergillus, such as Aspergillus niger, have been known to synthesize a wide variety of pigments, such as aspergillin, asperenone, azaphilones (azanigerones A–F), and melanin (Figure 5a) ^{[25][89][90][91]}[25,110,114,115]. Aspergillus nidulans was reported to produce ascoquinone A, norsolorinic acid, and melanin ^[25][92][93][25,112,113], whereas Aspergillus fumigatus was reported to produce melanin and melanin-like pigments ^[25][94][25,111]. In addition, a variety of other pigments such as asperenone, anishidiol, neoaspergillic acid, sterigmatocystin, and an uncharacterized yellow pigment have been discovered from Aspergillus nishimurae, Aspergillus awamori, Aspergillus sclerotiorum, Aspergillus versicolor, and Aspergillus terreus, respectively ^{[25][73][89][95][96]}[25,91,110,116,118]. Many other species of Aspergillus such as Aspergillus glaucus, Aspergillus cristatus, and Aspergillus repens have been reported to produce a variety of hydroxyanthraquinone pigments, emodin, physcion, questin, erythroglaucin, catenarin, and rubrocristin; while Aspergillus melleus, Aspergillus ochraceus, Aspergillus sulphureus, and Aspergillus westerdijkiae have been described to be major producers of polyketide-based pigments (rubrosulfin, viomellein, viopurpurin, and xanthomegnin) (Figure 5a) [25]. In addition to this, other pigments such as ferriaspergillin, ferrineoaspergillin, and an uncharacterized yellow pigment have also been reported from the genus Aspergillus (Figure 5a) ^[97][98][119,120]. Certain teleomorphic species of Aspergillus have been described as producers of a variety of pigments. Some of the well-known azaphilone pigments such as falconensins A–H, zeorin, falconensones A1 and B2 have been reported from Emericella falconensis and Emericella fruticulosa (currently known as Aspergillus falconensis and Aspergillus fruticulosus, respectively), epurpurins A-C from Emericella purpurea (currently known as Aspergillus purpureus), and the pigment sterigmatocystin from Emericella rugulosus, Emericella parvathecia, and Emericella nidulans (currently known as Aspergillus rugulosus, Aspergillus parvathecia, and Aspergillus nidulans) (Figure 5c). Similarly, other Aspergillus spp. such as Aspergillus amstelodami, Aspergillus chevalieri, Aspergillus glaucus, Aspergillus umbrosus, Aspergillus spiculosus, Aspergillus glaber, Aspergillus echinulatum, Aspergillus tonophilus, Aspergillus intermedius, Aspergillus leucocarpus, Aspergillus ruber, and Aspergillus cristatus (which were formerly known as Eurotium amstelodami, Eurotium chevalieri, Eurotium herbariorum, Eurotium umbrosum, Eurotium spiculosum, Eurotium spiculosum, Eurotium echinulatum, Eurotium tonophilum, Eurotium intermedium, Eurotium leucocarpum, Eurotium rubrum, and Eurotium cristatum, respectively) have also been reported to produce pigments such as physcion, erythroglaucin, flavoglaucin, auroglaucin, catenarin, rubrocristin, and emodin (Figure 5b) [25]. Members of different genera of the fungal family Pleosporaceae (Alternaria, Curvularia, Pyrenophora, etc.) have immense potential for pigment production. Species of Alternaria such as Alternaria alternata, Alternaria solani, Alternaria porri, and Alternaria tomatophila have been reported to produce a variety of pigments such as dactylariol, alterperylenol, dihydroalterperylenol, alternariol, alternariol-5-methyl ether, altenuene, alternarienoic acid, tenuazoic acid, stemphyperylenol, and altersolanol A (Figure 6) ^{[25][99][100][101]}[25,76,77,78]. Also, other members of the Pleosporaceae, Curvularia and Pyrenophora, have been known to produce different types of pigments, e.g., Curvularia lunata produces hydroxyanthraquinone pigments such as chrysophanol, cynodontin, helminthosporin, erythroglaucin, and catenarin, whereas different species of Pyrenophora such as Pyrenophora teres, Pyrenophora graminea, Pyrenophora tritici-repentis, Pyrenophora grahamii, Pyrenophora dictyoides, and Pyrenophora chaetomioides (which were previously known as Drechslera teres, Drechslera graminea, Drechslera tritici-repentis, Drechslera phlei, Drechslera dictyoides, Drechslera avenae, respectively) have also been reported to produce hydroxyanthraquinone pigments such as cynodontin, erythroglaucin, catenarin, helminthosporin, and tritisporin (Figure 6) ^[25][69][25,61]. Trichoderma, a well-known bio-control agent, has been known to produce a variety of pigments ^[25][102][25,124]. Several hydroxyanthraquinones such as pachybasin, chrysophanol, emodin, T22 azaphilone, 1-hydroxy-3-methyl-anthraquinone, 2,4,5,7-tetrahydroxyanthraquinone, 1,3,6,8-tetrahydroxyanthraquinone, and 1,8-dihydroxy-3-methyl-anthraquinone, have been reported from different species of Trichoderma (Trichoderma harzianum, Trichoderma polysporum, Trichoderma viride, and Trichoderma aureoviride) (Figure 7a) [25], whereas Trichoderma afrharzianum, Trichoderma pyramidale, and Trichoderma sp. 1 are reported to produce uncharacterized yellow pigments in submerged fermentation ^[77][71]. Studies have also revealed that certain species of Neurospora, such as Neurospora crassa, Neurospora sitophila, and Neurospora intermedia produce a variety of carotenoids such as phytoene, β-carotene, γ-carotene, lycopene, neurosporene, and neurosporaxanthin (Figure 7b) ^{[25][26][103]}[25,26,90]. Many genera of the Xylariaceae family, such as Daldinia, Hypoxylon, Jackrogersella, etc., have a great capability to synthesize pigments of very diverse colors and hues [25]. A variety of interesting pigments such as BNT (1,1ˊ-Binaphthalene-4,4ˊ-5,5́-tetrol), daldinol, daldinal A–C, and daldinin A–C have been reported from different species of Daldinia, such as Daldinia bambusicola, Daldinia caldariorum, Daldinia concentrica, Daldinia eschscholzii, Daldinia childiae, Daldinia clavata, Daldinia fissa, Daldinia grandis, Daldinia lloydi, Daldinia loculata, Daldinia petriniae, Daldinia singularis (Figure 8a). Similarly, several cohaerin variants (cohaerin A–K), multiformin A, and sassafrins D have been obtained from Jackrogersella cohaerens (formerly known as Annulohypoxylon cohaerens) (Figure 8a). Besides this, several species of Hypoxylon were declared to produce diverse pigments e.g., Hypoxylon fragiforme (hypoxyxylerone, cytochalasin H, fragiformins A–B, and mitorubrin), Hypoxylon howeanum (mitorubrin and azaphilones), Hypoxylon lechatii (vermelhotin and hypoxyvermelhotins A–C), Hypoxylon fuscum (daldinin A–C), Hypoxylon fulvo-sulphureum (mitorubrinol derivatives), Hypoxylon sclerophaeum (hypoxylone), Hypoxylon rickii (rickenyl B and D), Hypoxylon lenormandii and Hypoxylon jaklitschii (lenormandins A-G), Hypoxylon rubiginosum (mitorubrin, rubiginosin, and hypomiltin) (Figure 8a). Members of the Chaetomiaceae family also exhibit potential of pigment production. Chaetomium cupreum has been mentioned to produce red azaphilone pigments, oosporein, rotiorinols A–C, rubrorotiorin, whereas Chaetomium globosum produces yellow azaphilone pigments (chaetoviridins A–D), chaetoglobin A–B, chaetomugilins A–F, and cochliodinol (Figure 8b). Production of parietin (hydroxyanthraquinone pigment) has also been revealed from the Achaetomium sp. (Figure 8b) [25]. Also, the genera belonging to the family Cordycipitaceae such as Torrubiella, Cordyceps, Beauveria, Hyperdermium, and Lecanicillium have been revealed to be promising producers of bioactive pigments, e.g., tenellin and bassianin are reported from Beauveria bassiana and Beauveria brongniartii (formerly known as Beauveria tenella), pyridovericin and pyridomacrolidin from Beauveria bassiana, torrubiellones A–D from the genus Torubiella, oosporein from Lecanicillium aphanocladii, whereas anthraquinone-related compounds are reported from Cordyceps farinosa (formerly known as Isaria farinosa) (Figure 9a) ^{[41][105][106][107][108]}[41,73,74,75,125]. Similarly, the pigments erythrostominone, 4-O-methyl erythrostominone, deoxyerythrostominone, deoxyerythrostominol, epierythrostominol, and 3,5,8-TMON (3,5,8-trihydroxy-6-methoxy-2-(5-oxohexa-1,3-dienyl)-1,4-naphthoquinone) have been reported from Ophiocordyceps unilateralis (formerly known as Cordyceps unilateralis), and skyrin from Hyperdermium bertonii (Figure 9a) [25]. Apart from this, studies have reported the production of the pigment xylindein from Chlorociboria aeruginosa and Chlorociboria aeruginascens, draconin red from Scytalidium cuboideum, and a yellow pigment from Scytalidiium ganodermophthorum and Scytalidium lignicola. Other pigments, such as orevactaene produced from Epicoccum nigrum, emodin, ω-hydroxyemodin, and emodic acid from Hamigera avellanea (formerly known as Talaromyces avellaneus) are also known (Figure 3b, Figure 9b) ^{[33][36][37][39][41][83]}[33,36,37,39,41,109]. Recently, fungi such as Sanghuangporus baumii and Clonostachys intermedia have been found to produce a yellow pigment under submerged fermentation ^[77][71]. Production of melanin was reported from different groups of fungi such as Phyllosticta capitalensis, Xylaria polymorpha, Trametes versicolor, Inonotus hispidus, Oxyporus populinus, Fomes fomentarius, Exophiala dermatitidis, Tuber melanosporum, Sporothrix schenckii, and Cryptococcus neoformans ^{[29][34][35][44][109][110][111]}[29,34,35,44,80,81,83]. Similarly, a study has shown the possible industrial application of the red pigment produced by Paecilomyces sinclairii ^[112][126]. Besides filamentous fungi, certain genera of yeasts (Rhodotorula, Sporidiobolus, Sporobolomyces and Xanthophyllomyces) have also been known as pigment producers. Different species of Rhodotorula (Rhodotorula glutinis, Rhodotorula mucilaginosa (syn. Rhodotorula rubra), Rhodotorula babjevae, Rhodotorula toruloides Rhodotorula graminis), Sporidiobolus (Sporidiobolus pararoseus, Sporidiobolus johnsonii), and Sporobolomyces (Sporobolomyces uberrimus, Sporobolomyces salmonicolor) have been reported to be prolific producers of torulin and torularhodin ^[113][127]. Researchers have discovered pigments such as β-carotene, torulene, and torularhodin from Rhodotorula glutini and multi-hydroxy carotenoids (4,4′-dihydroxy-nostoxanthin and 4-hydroxy-nostoxanthin) from Xanthophyllomyces dendrorhous (Figure 10) ^[13][114][13,128]. In addition to terrestrial fungi, marine fungi are also very good producers of a variety of unique pigments having promising therapeutic and industrial applications ^[115][116][129,130]. Studies on marine fungi by many researchers have reported a wide range of pigments and hues, e.g., a variety of anthraquinone pigments [asperflavin, 2-O-methyleurotinone, questin, eurorubrin, 2-O-methyl-9-dehydroxyeurotinone, 2-O-methyl- 4-O-(α-D-ribofuranosyl)-9-dehydroxyeurotinone, and 6, 3-O-(α-D-ribofuranosyl)-questin] from the mangrove endophytic fungus A. ruber (formerly known as Eurotium rubrum), fusarnaphthoquinones B and fusarnaphthoquinones C from the sea fan-derived fungi Fusarium species, and bianthraquinone derivatives (alterporriol K, alterporriol L, and alterporriol M) from mangrove endophytic Alternaria sp. (Figure 11) ^{[117][118][119]}[69,79,117]. Researchers have also investigated the red pigment production from mangrove fungus Penicillium sp. and a yellow pigment production from the marine sponge-associated fungus Trichoderma parareesei ^[120][121][70,99]. Also, many studies have revealed the production of polyketide pigments (N-threonine rubropunctamine) and chlorinated azaphilone pigments (chaephilone-C, chaetoviridides-A, chaetoviridides-B, chaetoviridides-C) from marine fungal isolates of Talaromyces spp. and Chaetomium sp., respectively (Figure 11) ^[122][123][72,82]. A recent study has reported a novel pigment, N-GABA-PP-V (6-[(Z)-2-Carboxyvinyl]-N-GABA-PP-V), along with N-threonine-monascorubramine, N-glutaryl-rubropunctamine, and PP-O from the marine-derived fungus Talaromyces albobiverticillius (Figure 11) ^[124][131]. Many antarctic fungi have also been discovered to produces pigments of different chemical classes and characteristics. A number of yeast and filamentous fungi isolated from the different samples collected from Antarctic regions have been reported to produce a variety of pigments with different colors ^[125][86].